The mother of a hostage American journalist pleaded for his release Wednesday in a video directed at the Islamic State group, while new images emerged of mass killings, including masked militants shooting kneeling men after the capture of a strategic air base in Syria.

Shirley Sotloff's plea came as a UN commission accused the group, which dominates a broad swath of territory spanning the Syria-Iraq border, of committing crimes against humanity and President Barack Obama weighs options for targeting the extremists' stronghold in Syria.